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Abstract  

The 4-nitrobenzyl ester of acetylphenylhydroxyacetic acid differs in its melting behaviour from other nitrobenzyl esters of phenylhydroxyacetic or acetylphenylhydroxyacetic acids, the racemate having a higher melting point than the enantiomers. By means of thermal analysis, IR spectroscopy and X-ray diffractometry the ester can be shown to occur in two crystalline modifications. In the process of solidification of the molten mass, at first a modification of higher energy is formed, obviously being caused by an excess of one enantiomer, which is then exothermally rearranged in the lattice of the racemate.

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Summary

Separation of enantiomers by liquid chromatography is a powerful method which has long been used to obtain enantiomerically pure compounds in the pharmaceutical, food, and agrochemical industries. Optimization of such separations to achieve high-performance resolution of pairs of enantiomers is a challenging task. To this end, mathematical models of adsorption on chiral stationary phases have been widely used to predict the performance of chromatographic columns packed with these materials. In this review we discuss the basic adsorption models used in chiral separations, and their extension to specific cases. We also outline combination of adsorption models with models describing mass-transport processes in a chromatographic column. We focus on the most popular chiral stationary phases used in chromatographic separations, for which we describe recent developments in theoretical modeling of enantioselective binding.

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Abstract  

The physicochemical characterization of the solid-state enantiomers and racemate of efaroxan hydrochloride (C13H17N2O+Cl-, M=252.5 g mol-1) was performed by thermoanalytical methods (differential scanning calorimetry, thermogravimetry and thermomicroscopy) and spectral methods (infrared spectrometry and X-ray diffractometry). The efaroxan enantiomers and racemate were shown to be unstable near the melting point. At the beginning of the decomposition, a loss of hydrogen chloride was observed. However when sealed pans were used, the compounds decomposed at higher temperature, allowing a precise evaluation of the melting enthalpies by means of differential scanning calorimetry. The nature of the racemate and its thermal stability were assessed by evaluating its free formation enthalpy. An enantiotropic solid-solid transformation (II→I) was noted for the racemate; the reverse process (I→II) follows zero-order kinetics.

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Ibuprofen is one of the most common nonsteroidal anti-inflammatory and analgesic drugs. It is marked as a racemic mixture though it is known that the pharmacological activity resides in the (+)-(S)-enantiomer only. The process of conversion of (+)-(S)-ibuprofen enantiomer into (−)-(R)-enantiomer, inactive to cyclooxygenase, in methanol and cyclohexane using a chiral selector in TLC separation was investigated. Based on the values of k, t 0.1, and t 0.5, it is shown that the interconversion of (+)-(S) into (−)-(R)-enantiomer runs 10 times faster in polar methanol than in lipophilic cyclohexane.

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Abstract  

It is shown from experiments on leucine, that is possible to obtain pure enantiomer tracers of amino acids by using radioactive racemates only. The resolution takes place in a single crystallization step after mixing the active racemate with the inactive enantiomer, due to an absolute stereoselection.

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Acta Chromatographica
Authors: M. Sajewicz, E. John, D. Kronenbach, M. Gontarska, and T. Kowalska

Summary

Enantiomer separation by TLC is still much less frequent than with other, mostly instrumental, chromatographic techniques. From a literature survey it is apparent that separation of the enantiomers of d,l-lactic acid is primarily of interest to the diary industry and that this particular separation is less frequently performed by chromatographic than by membrane techniques. As far as we are aware, before our studies only one report of TLC separation of the enantiomers of d,l-lactic acid was available in the literature; this is dated 1991 and describes the use of non-instrumental TLC only. In this study, we started by reproducing the TLC procedure originating from 1991, for this purpose using TLC with automatic sample application and densitometric detection. We managed to repeat the earlier procedure and to achieve full, i.e. baseline, separation of the enantiomers, with a remarkable distance between the two antimers. However, we revealed a significant drawback of this separation procedure — d-(−)-lactic acid was transported almost with the mobile-phase front and its densitometric quantification was barely possible because of the relatively high UV absorption of the mobile-phase front line. The reference method for separation of the enantiomers of d,l-lactic acid consisted in preliminary impregnation of commercial silica gel TLC plates with copper(II) acetate. In-situ formation of bidentate complexes of the d,l-lactic acid antimers with the Cu2+ cation resulted in different mobilities of these complex cations in the planar chromatographic system. The objectives of this study were twofold — to investigate separation of the enantiomers of d,l-lactic acid with other transition metal cations (i.e., Co2+, Ni2+, and Mn2+) used to impregnate the silica gel (to achieve resolution that might enable quantification of the two lactic acid antimers and not only the l-(+) enantiomer) and to gain deeper insight into the mechanism of separation with these metal cations. For purposes of comparison, we chromatographed d,l-lactic acid on non-impregnated silica gel layers. As a result, we managed to establish efficient separation conditions with the Ni2+ and Co2+ cations that outperformed the earlier established procedure involving the Cu2+ cation, and — partially at least — to elucidate the mechanism of separation of the enantiomers of d,l-lactic acid by these TLC systems. The Mn2+ cation proved unsuitable for the purpose. Finally, we managed to separate the enantiomers of d, l-lactic acid on non-impregnated silica gel layer also, which seems yet more proof of the microcrystalline chirality of silica gel used as stationary phase and of its substantial contribution to the enantiomer separation investigated.

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Pressurized planar electrochromatography (PPEC) with commercially available chiral plates has been used for separation of the enantiomers of tryptophan and valine. The effects on migration distance of polarization voltage, buffer pH and concentration, and the concentration of the organic component of the mobile phase have been investigated. Solute separation is compared for PPEC and conventional planar chromatography.

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Summary

Enantiomeric resolution of two commonly used β-blockers, namely, (±)-propranolol and (±)-atenolol, has been achieved on silica gel layers which were bulkimpregnated with β-cyclodextrin. Solvent systems DMF-ethyl acetate-butanol (3:2:5, υ/υ) and butanol-acetic acid-ethyl acetate-ammonia (5:2:2:0.5, υ/υ) successfully resolved the enantiomers of (±)-propranolol and (±)-atenolol, respectively. The spots were located with iodine vapor. The effects of concentration of the chiral selector and mobile phase variation were also studied.

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A method for chromatographic separation of the enantiomers of (±)-metoprolol tartrate (±MeT) employing a chiral mobile phase additive (CMPA) is described. By using silica gel plates previously impregnated with the mobile phase (ethanol-water, 70 + 30, v/v ) containing D -(−)-tartaric acid as chiral selector, direct separation of the enantiomers of ±MeT was achieved. The results of experiments with different concentrations of D -(−)-tartaric acid (5.8, 11.6, and 23 mmol L −1 ) revealed that the best resolution of the enantiomers of ±MeT was achieved with 11.6 mm D -(−)-tartaric acid in both the mobile phase and the impregnation solution, at 25 ± 2°C.Spot visualization on chromatograms was performed by use of a fixed-wavelength ( λ = 254 nm) ultraviolet lamp or an iodine vapor-saturated chamber.

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Resolution of racemic metoprolol, propranolol, carvedilol, bisoprolol, salbutamol, and labetalol, commonly used β-blockers, into their enantiomers has been achieved by TLC on silica gel plates impregnated with optically pure L -Glu and L -Asp. Acetonitrile-methanol-water-dichloromethane and acetonitrile-methanol-water-glacial acetic acid mobile phases in different proportions enabled successful separation. The spots were detected with iodine vapor. The detection limits were 0.23, 0.1, 0.27, 0.25, 0.2, and 0.2 μg for each enantiomer of metoprolol, propranolol, carvedilol, bisoprolol, salbutamol, and labetalol, respectively.

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